1. Field of the Invention
The present invention relates to a trunk apparatus and a terminal apparatus used in a wavelength-division-multiplexing communication system and a monitoring and controlling method for the wavelength-division-multiplexing communication system.
2. Description of the Related Art
In a recent optical communication, an optical transmission system using a WDM (wavelength-division-multiplexing) technology has been put to practical use. FIGS. 1A, 1B and 1C show structures of such a system. FIG. 1A shows a case of a simplified structure; FIG. 1B shows a case in which an ADM (add/drop multiplexer) is used in the system; and FIG. 1C shows a case of a ring-type structure.
In the case of the simplified structure shown in FIG. 1A, a transmitting terminal 101 wavelength-division-multiplexes a transmission signal, and sends the signal to an optical cable. Trunk apparatuses 102 to 104 relay the signal so as to transmit the signal to a receiver terminal 105.
In the case of FIG. 1B in which ADMs are present, a WDM communication system (a) and a WDM communication system (b) are interconnected by ADMs 108 and 113. The ADM 108 drops a part of a signal from the transmitter terminal 106 to the ADM 113, and adds a signal from the ADM 113. The ADM 113 drops a part of a signal from the transmitter terminal 111 to the ADM 108, and adds a signal from the ADM 108. Thereby, intercommunication between the WDM communication system (a) and the WDM communication system (b) is achieved.
In the case of the ring-type structure shown in FIG. 1C, nodes 116A to 116D to which a terminal apparatus can be connected and ADMs 118A to 118D together form a ring-type network.
It should be noted that although the transmitter terminal apparatus or the receiver terminal apparatus is represented as a terminal apparatus, a single terminal apparatus may serve as both a transmitter terminal apparatus and a receiver terminal apparatus.
FIG. 2 shows a structure of a terminal apparatus of a WDM communication system. The terminal apparatus 119 wavelength-division-multiplexes signals sent from apparatuses 120 to 122, each apparatus being a SDH (synchronous digital hierarchy) terminal multiplexing apparatus or a SONET (synchronous optical network) terminal multiplexing apparatus, and sends the signals to a transmission path 135. The terminal apparatus 119 also wavelength-division-demultiplexes signals received via a transmission path 136, and sends the signals to the apparatuses 120 to 122. In the terminal apparatus 119, signals of wavelengths xcex1 to xcexn are multiplexed by the transmitter-side wavelength division multiplexing apparatus 125 via a variable light attenuating apparatus 124. The multiplexed signal is amplified by a transmitter-side multi-wavelength amplifier 127, and is output to the transmitter-side transmission path 135. A WDM signal received via the receiver-side transmission path 136 is amplified by a receiver-side multi-wavelength amplifier 133. The amplified signal is demultiplexed into signals of each wavelength and is transmitted to the apparatuses 120 to 122. It should be noted that dispersion compensation fiber modules 126 and 134 are provided for compensating for a dispersion in an optical cable. Boosters 128 and 132 are booster units for additional exiting-light sources. An analyzer 129 is for a spectrum analysis. Each part in the terminal apparatus 119 is controlled by a central monitoring and controlling apparatus 123.
FIG. 3 shows an example of a structure which does not includes an ADM. A signal transmitted via a transmission path 152 is amplified by a multi-wavelength amplifier 143, and the amplified signal is sent to a transmission path 153. Similarly, a signal transmitted via a transmission path 154 is amplified by a multi-wavelength amplifier 146, and the amplified signal is sent to a transmission path 155. A monitoring and controlling apparatus 151 including monitoring and controlling units 144 and 145 is connected to the multi-wavelength amplifiers 143 and 146 so as to monitor and control the WDM communication system.
FIG. 4 shows an example of a structure which includes an ADM. A signal transmitted via the transmission path 152 is amplified by a multi-wavelength amplifier 159, and input to an ADM 158. An output signal of the ADM 158 is amplified by a multi-wavelength amplifier 161, and the amplified signal is sent to the transmission path 153. Similarly, a signal transmitted via the transmission path 154 is amplified by a multi-wavelength amplifier 162, and input to the ADM 158. An output signal of the ADM 158 is amplified by a multi-wavelength amplifier 160, and the amplified signal is sent to the transmission path 155. A signal of a wavelength xcexa on a transmission path 156 is added by the ADM 158, and a signal of a wavelength xcexd is dropped to a transmission path 157 by the ADM 158. It should be noted that dispersion compensation fiber modules 140 and 148 are provided for compensating for dispersion in an optical cable. BST boosters 141 and 147 are booster units for additional exiting-light sources. Each part in the apparatus is controlled by a central monitoring and controlling apparatus 149.
FIG. 5 shows an example of a structure of an ADM (passive) having no switch circuit. A wavelength-division-multiplex signal transmitted through a transmission path 163 is wavelength-division-demultiplexed by a wavelength-division-demultiplexing apparatus 164. A signal of a wavelength xcexd is dropped from among the demultiplexed wavelengths, and the rest of the wavelengths are input to a wavelength-division-multiplexing apparatus 165. Additionally, a signal of a wavelength xcexa is added to the wavelength-division-multiplexing apparatus 165. Signals of the wavelengths including the wavelength xcexa are wavelength-division-multiplexed by the wavelength-division-multiplexer 165, and the multiplexed signal is output to a transmission path 166.
FIG. 6 shows an example of a structure of an ADM (active) having a switch circuit. A plurality of switch circuits 172 to 177 are provided so as to select wavelengths to be dropped. The remaining structure is the same as the ADM shown in FIG. 5.
FIG. 7 shows an example of a structure of a monitoring and controlling apparatus 180. A monitoring and controlling signal having 1 to n channels is input via a transmission path 181, and is converted into an electric signal by an optoelectric converter 183. The signal is input to a serial/parallel converter 187 via a frame end unit 186, and is subjected to a serial/parallel conversion so as to obtain a monitoring and controlling signal 193. On the other hand, a monitoring and controlling signal 194 having 1 to n channels is input to a parallel/serial converter 189 and is converted into a serial signal. Then, a frame for synchronization is added to the serial signal by a frame generating unit 188, and the serial signal is converted into an optical signal by an electrooptic converter 184. The optical signal is transmitted through a transmission path 182.
It should be noted that a clock received by the optoelectric converter 183 is supplied to the frame synchronization circuit 185, and a frame pulse is supplied from the frame synchronization circuit 185 to the frame end unit 186. Additionally, the clock received by the optoelectric converter 183 is also supplied to a selector 192. The selector 192 selects one of the clock, an output of an oscillator 191 and clocks 195 received from outside. The selected clock is supplied to a pulse-generating unit 190. An output of the pulse-generating unit 190 is supplied to the parallel/serial conversion circuit 189 and the frame-generating unit 188.
FIG. 8 is an illustration of a simplified structure shown in FIG. 1A which is further simplified and represented in view of a wavelength. A plurality of modulated light signals having different wavelengths (xcex1 to xcex4) are multiplexed by a transmission-side terminal apparatus 1, and transmitted through a single optical fiber 4. The optical fiber transmission path 4 is provided with an optical amplifier 2. The optical amplifier 2 amplifies the light signals (xcex1 to xcex4) at the same time, and transmits the light signals to a receiver-side terminal apparatus 3. The receiver-side terminal apparatus 3 demultiplex the multiplexed signal, and distributes the light signals on an individual wavelength basis.
Since the multi-wavelength optical amplifier 2 simply amplifies a light signal, an output level of the optical amplifier 2 is ordinarily controlled to be constant by an ALC (automatic level control). Accordingly, if, for example, the light signal having the wavelength xcex2 is cut off, the optical amplifier 2 increases the amplification of other light signals having the wavelengths xcex1, xcex3 and xcex4 so as to maintain the total output level. As a result, the light signals having the wavelengths xcex1, xcex3 and xcex4 are unnecessarily amplified. This may cause interference between the light signals of the different wavelengths or a sharp change in a level of a received light signal at a receiver, resulting in an error in the transmitted information. Especially, since the interference between signals of different wavelengths continuously occurs, it may become difficult to perform communication when the interference is serious.
In order to eliminate such a problem in a control using an ALC technique, there is a method for detecting a cut-off of a light signal by providing a photodiode to an optical amplifier for each signal of an individual wavelength. However, this method creates another problem in that a number of monitors at a final stage must be determined and provided at a design stage for future use since a number of monitors equal to a number of signals of different wavelengths to be multiplexed must be provided, and an increase in the number of signals of different wavelengths to be multiplexed is uneconomical.
Additionally, there is a method in which a monitoring signal is provided for signals of each wavelength. That is, a signal of each wavelength is frequency-modulated by a modulation different from a modulation according to a signal. The frequency of the frequency modulation is different for signals of each wavelength. In an optical amplifier, a single photodiode is provided to receive all the signals of the different wavelengths. The received signals of different wavelengths are split by a filter so as to obtain each frequency-modulated wave. The frequency-modulated wave is demodulated so as to detect a cut-off of a signal of each wavelength. In this case, a filter circuit must be designed in consideration of a number of signals of different wavelengths to be used.
Considering the above-mentioned situation, a method using an optical monitoring line has been put to use as a method for controlling a gain of an optical amplifier in a WDM communication field. That is, an optical line for monitoring and controlling is provided separately from a signal transmission path so as to provided information of a signal currently input to each optical amplifier by sending the monitoring and controlling signal through the monitoring line. Each optical amplifier controls a gain in accordance with the information.
In this case, if a number of signals of different wavelengths to be used is increased or decreased during practical use, such information is provided to each optical amplifier through the monitoring line. Accordingly, each optical amplifier can be shifted to a gain control state responsive to a new number of wavelengths. Additionally, if a signal of a wavelength being used is cut off during an operation, such a cut-off can be detected by a transmitter-side terminal apparatus and the transmitter-side terminal apparatus may notify each optical amplifier of the cut-off. Thus, each optical amplifier may control a gain in accordance with such information.
Additionally, as shown in FIGS. 4 to 6, an optical amplifier is able to have an optical ADM function using wavelength selectivity, and such an ADM is used in a ring-type network. FIG. 9 shows an example of a structure of a WDM communication system using an optical amplifier having an ADM function. An optical amplifier 15 drops signals of wavelengths input from an optical transmission path 17, and adds signals of new wavelengths 6. Accordingly, the number of signals of different wavelengths at the optical amplifier 16 and supplied to a transmitter-side terminal apparatus 13 is a sum of the number of signals of wavelengths 6 added by the optical amplifier A 16 and the number of signals of wavelengths 7 transmitted from a transmitter-side terminal apparatus 11.
In a conventional technique in which a gain control is performed by transmitting a monitoring and controlling signal for an optical amplifier through a monitoring line, there are the following problems.
(1) When a number of signals of different wavelengths is changed, such information is provided to each optical amplifier via the monitoring line. Each optical amplifier shifts to a gain control state responsive to a new number of signals of different wavelengths in accordance with the information. However, if an added signal of a new wavelength reaches each optical amplifier before the information notifying of an increase in the number of signals of different wavelengths reaches each optical amplifier, a power of a signal of each wavelength is decreased due to an ALC control. This decrease may last for a very short time, but a burst error may be generated.
(2) If a signals of certain wavelength is cut off, a transmitter-side terminal apparatus provides such information to each optical amplifier via the monitoring line. Each optical amplifier controls a gain according to the information. However, it takes a certainiperiod after the signal of the certain wavelength is cut off for the terminal apparatus to detect the cut-off and provide the information to each optical amplifier via the monitoring line. During this period, each optical amplifier does not recognize the fact that the signal of the certain wavelength was cut off. Thus, a power of signal of the other wavelengths is increased for a short time due to ALC. The period is very short, but a burst error may be generated.
(3) If an optical ADM function using optical wavelength selectivity is provided to an optical amplifier, a number of signals of different wavelengths input to the optical amplifier is not always equal to a number of signals of different wavelengths transmitted from a terminal apparatus. Accordingly, a number of signals of different wavelengths is monitored at a certain points in the network so as to provide information of a number of signals of different wavelengths input to each optical amplifier. In the case of FIG. 9, if a signal of a wavelength added by the optical amplifier 15 is cut off, a number of signals of different wavelengths input to the optical amplifier 16 is changed. However, since the transmitter-side terminal apparatus 13 merely manages the number of signals of different wavelengths output therefrom, information indicating the fact that the signals of the wavelength added by the optical amplifier 15 was cut off cannot be provided to the optical amplifier 16. Accordingly, a power of each signal of each wavelength at the optical amplifier 16 is increased due to the ALC, and a transmission error may be generated.
It is a general object of the present invention to provide an improved and useful trunk apparatus used in a WDM communication system in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide a trunk apparatus which prevents a signal error due to a fluctuation in an optical power in a WDM communication system.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention a trunk apparatus of a wavelength-division-multiplex communication system comprising:
an optical amplifier;
a transmission power monitor for monitoring a transmission power of the trunk apparatus;
a monitoring and controlling signal transmitting and receiving circuit for transmitting and receiving a monitoring and controlling signal; and
an automatic gain control circuit for performing an automatic gain control with respect to the optical amplifier,
wherein the monitoring and controlling signal transmitting and receiving circuit sends transmission power information regarding the transmission power monitored by the transmission power monitor to an immediately proceeding trunk apparatus located on a downstream side of the local trunk apparatus;
the monitoring and controlling signal transmitting and receiving circuit receives transmission power information sent from an immediately preceding trunk apparatus located on an upstream side of the local trunk apparatus, and sends the received transmission power information to the automatic gain control circuit; and
the automatic gain control circuit controls the optical amplifier based on the transmission power information received from the immediately preceding trunk apparatus so as to equalize the transmission power of the local trunk apparatus to the transmission power of the immediately preceding trunk apparatus.
According to the above-mentioned invention, the trunk apparatus of a wavelength-division-multiplex communication system sends a transmission power information to the immediately proceeding trunk apparatus. Additionally, the trunk apparatus performs an automatic gain control with respect to the optical amplifier based on received transmission power so that the transmission power of the trunk apparatus is equalized to a transmission power information of the immediately preceding trunk apparatus. Accordingly, if a number of signals of different wavelengths to be transmitted is changed, the transmission power can be maintained to be constant. Thus, there is no fluctuation in the transmission power, which results in prevention of a signal error due to fluctuation in an optical power.
The trunk apparatus according to the present invention may further comprise:
a reception power monitor for monitoring a reception power; and
a loss calculating circuit for calculating a loss of a transmission path between the local trunk apparatus and the immediately preceding trunk apparatus,
wherein the loss calculating circuit calculates the loss of the transmission path between the local trunk apparatus and the immediately preceding trunk apparatus based on an output of the reception power monitor and the transmission power information received from the immediately preceding trunk apparatus; and
the automatic gain control circuit controls the optical amplifier based on a result of the calculation of the loss calculating circuit so as to compensate for the loss of the transmission path between the local trunk apparatus and the immediately preceding trunk apparatus.
According to this invention, the trunk apparatus sends transmission power information to the immediately proceeding trunk apparatus. Additionally, the trunk apparatus obtains transmission power information from the immediately preceding trunk apparatus so as to calculate a loss of the transmission path between the trunk apparatus and the immediately preceding trunk apparatus. The trunk apparatus performs an automatic gain control with respect to the optical amplifier so that the loss of the transmission path between the local trunk apparatus and the immediately preceding trunk apparatus is compensated for. Thus, the transmission power of the trunk apparatus is equalized to a transmission power of the immediately preceding trunk apparatus. Accordingly, if a number of signals of different wavelengths to be transmitted is changed, the transmission power can be maintained to be constant. Thus, there is no fluctuation in the transmission power which results in prevention of a signal error due to fluctuation in an optical power.
The above-mentioned trunk apparatus may further comprise:
a direct-current light source having a predetermined transmission power; and
means for transmitting an output of the direct-current light source to the immediately proceeding trunk apparatus,
wherein the loss calculating circuit calculates the loss of the transmission path between the local trunk apparatus and the immediately preceding trunk apparatus based on the output of the reception power monitor for a direct-current light source of the immediately preceding trunk apparatus and information regarding a transmission power of the direct-current light source of the immediately preceding trunk apparatus, instead of calculating the loss based on the output of the reception power monitor and the transmission power information of the immediately preceding trunk apparatus.
According to this invention, the direct current light source is exclusively provided to measure a reception power. Since a transmission power of the direct-current light source does not fluctuate, an accurate calculation of the loss of the transmission path between the trunk apparatus and the immediately preceding trunk apparatus can be achieved.
Additionally, in the above-mentioned trunk apparatus, the transmission power monitor may monitor the direct-current light source;
the monitoring and controlling signal transmitting and receiving circuit may send transmission power information regarding a transmission power of the direct-current light source monitored by the transmission power monitor to the immediately proceeding trunk apparatus; and
the loss calculating circuit may calculate the loss of the transmission path between the local trunk apparatus and the immediately preceding trunk apparatus based on the output of the reception power monitor for the direct-current light source of the immediately preceding trunk apparatus and information regarding the transmission power of a direct current light source of the immediately receding trunk apparatus supplied by the monitoring and controlling signal transmitting and receiving circuit.
Accordingly, even when there is a dispersion in a power of the direct current light source of the immediately preceding trunk apparatus, the loss of the transmission path between the trunk apparatus and the immediately preceding trunk apparatus can be accurately calculated since the transmission power information of the direct-current light source provided in the immediately preceding trunk apparatus is used for the calculation of the loss.
Additionally, the trunk apparatus according to the present invention may further comprise:
a monitoring and controlling signal light having a predetermined transmission power; and
means for transmitting the monitoring and controlling signal light to the immediately proceeding trunk apparatus,
wherein the loss calculating circuit calculates the loss of the transmission path between the local trunk apparatus and the immediately preceding trunk apparatus based on the output of the reception power monitor for the monitoring and controlling signal light of the immediately preceding trunk apparatus and information regarding a transmission power of the monitoring and controlling signal light of the immediately preceding trunk apparatus, instead of calculating the loss based on the output of the reception power monitor and the transmission power information of the immediately preceding trunk apparatus received by the monitoring and controlling signal transmitting and receiving apparatus.
According to this invention, the monitoring and controlling signal light of the immediately preceding trunk apparatus has a known power level, and the loss of, the transmission path between the trunk apparatus and the immediately preceding trunk apparatus is calculated based on the reception power of the monitoring and controlling signal light of the immediately preceding trunk apparatus. Thus, there is no need to send transmission power information by the monitoring and controlling signal.
In the above-mentioned trunk apparatus, the transmission power monitor may monitor the monitoring and controlling signal light;
the monitoring and controlling signal transmitting and receiving circuit may send transmission power information regarding a transmission power of the monitoring and controlling signal light monitored by the transmission power monitor to the immediately proceeding trunk apparatus; and
the loss calculating circuit may calculate the loss of the transmission path between the local trunk apparatus and the immediately preceding trunk apparatus based on the output of the reception power monitor for the monitoring and controlling signal light of the immediately preceding trunk apparatus and information regarding the transmission power of the monitoring and controlling signal light of the immediately preceding trunk apparatus supplied by the monitoring and controlling signal transmitting and receiving circuit.
Accordingly, the transmission power information of the monitoring and controlling signal light is sent to the immediately proceeding trunk apparatus. Thus, when a power of the monitoring and controlling signal light of the immediately preceding trunk apparatus fluctuates, the trunk apparatus can accurately calculate the loss of the transmission path between the trunk apparatus and the immediately preceding trunk apparatus.
Additionally, a power level of the monitoring and controlling signal light may be periodically set to a constant level. Accordingly, a reception power of the monitoring and controlling signal light can be measured in an area of the light where the power level is constant. Thus, the loss of the transmission path between the trunk apparatus and the immediately preceding trunk apparatus can be accurately calculated.
Additionally, the trunk apparatus according to the present invention may be provided with an add-drop-multiplexer. Further, the trunk apparatus according to the present invention may be provided with a memory unit for storing information regarding the automatic gain control.
Additionally, there is provided according to another aspect of the present invention a trunk apparatus of a wavelength-division-multiplex communication system having an add and drop multiplexing function, the trunk apparatus comprising:
an optical amplifier;
a transmission power monitor for monitoring a transmission power of the trunk apparatus;
a monitoring and controlling signal transmitting and receiving circuit for transmitting and receiving a monitoring and controlling signal; and
an automatic gain control circuit for performing an automatic gain control with respect to the optical amplifier,
wherein the monitoring and controlling signal transmitting and receiving circuit sends transmission power information to an immediately proceeding trunk apparatus located on a downstream side of the local trunk apparatus, the transmission power information regarding a total transmission light except for a monitoring and controlling signal light of the local trunk apparatus;
the monitoring and controlling signal transmitting and receiving circuit receives transmission power information sent from an immediately preceding trunk apparatus located on an upstream side of the local trunk apparatus, and sends the received transmission power information to the automatic gain control circuit; and
the automatic gain control circuit controls the optical amplifier based on the transmission power information received from the immediately preceding trunk apparatus so as to equalize the transmission power of the local trunk apparatus to the transmission power of the immediately preceding trunk apparatus.
According to this invention, when a number of signals of different wavelengths is changed according to the add and drop multiplexing function, the power of the signal received by the trunk apparatus is always at a constant level. Thus, generation of a signal error due to a fluctuation in the optical power can be prevented.
The above-mentioned trunk apparatus may be provided with a memory unit for storing information regarding the automatic gain control.
Accordingly, when a failure occurs in the monitoring and controlling signal or a light source for measurement, a normal operation can be resumed by using information stored in the memory. Thus, an erroneous operation of the optical amplifier can be prevented, and there is no situation in which communication cannot be performed using signals of different wavelengths. Additionally, the monitoring and controlling signal transmitting and receiving circuit can be replaced without problems.
Additionally, there is provided according to another aspect of the present invention a method for monitoring and controlling a wavelength-division-multiplex communication system, comprising the steps of:
collecting present reception power information and amplification limit value information with respect to all optical amplifiers through which a signal light having an additional wavelength passes, the signal light of the additional wavelength being added to signal lights of wavelengths being used, the collection of information being performed before a light source of the signal light of the additional wavelength is turned on:
determining whether or not the signal light of the additional wavelength can be added without decreasing a power of each of the signal lights of wavelengths being used based on the collected information;
turning on the light source when the signal light of the additional wavelength can be added without decreasing the power of each of the signal light of the wavelengths being used; and
prohibiting an addition of the additional wavelength when the signal lights of the additional wavelength cannot be added without decreasing the power of each of the signal lights of the wavelengths being used.
According to the above-mentioned invention, the number of signal lights of wavelengths to be used is prevented from being excessively increased. Thus, the trunk apparatus can maintain an appropriate automatic gain control function.
Additionally, there is provided according to another aspect of the present invention a method for monitoring and controlling a wavelength-division-multiplex communication system, comprising the steps of:
transmitting a signal of an additional wavelength added to signals of wavelengths being used by gradually increasing a power of the signal of the additional wavelength, an addition of the signal of the additional wavelength being made at a terminal apparatus;
sending an alarm from a trunk apparatus to all directions when a power level of a signal received by the trunk apparatus exceeds a limit of amplification of an optical amplifier provided in the trunk apparatus, the limit of amplification being previously set to a small value having a margin; and
canceling an addition of the signal of the additional wavelength by immediately decreasing a power of the signal of the additional wavelength when the terminal apparatus receives the alarm sent from the trunk apparatus.
According to the present invention, the number of signals of the wavelengths to be used is prevented from being excessively increased even when a path through which a signal of a wavelength to be added passes is unknown. Thus, the trunk apparatus can maintain an appropriate automatic gain control function.
Additionally, there is provided according to another aspect of the present invention a method for monitoring and controlling a wavelength-division-multiplex communication system, comprising the steps of:
sending transmission power information to an immediately proceeding trunk apparatus located on a downstream side, the transmission power information being sent from one of a transmitting terminal apparatus and trunk apparatuses; and
performing an automatic gain control with respect to an optical amplifier provided in each of the trunk apparatuses based on transmission power information received from an immediately preceding trunk apparatus so as to equalize a transmission power of each trunk apparatus to a transmission power of the immediately preceding trunk apparatus.
Additionally, there is provided according to another aspect of the present invention a terminal apparatus of a wavelength-division-multiplex communication system comprising:
a monitoring and controlling signal transmitting and receiving circuit for transmitting and receiving a monitoring and controlling signal; and
a light source for a signal light of an additional wavelength to be added to signal lights of wavelengths being used,
wherein the monitoring and controlling signal transmitting and receiving circuit collects present reception power information and amplification limit value information with respect to all optical amplifiers through which the signal light having the additional wavelength passes, the collection of information being performed before the light source of the signal light of the additional wavelength is turned on; and
the monitoring and controlling signal transmitting and receiving circuit determines whether or not the signal light of the additional wavelength can be added without decreasing a power of each of the signal lights of the wavelengths being used based on the collected information so as to turn on the light source only when the signal light of the additional wavelength can be added without decreasing the power of each of signal lights of the wavelengths being used.
Additionally, there is provided according to another aspect of the present invention a terminal apparatus of a wavelength-division-multiplex communication system comprising:
a monitoring and controlling signal transmitting and receiving circuit for transmitting and receiving a monitoring and controlling signal; and
a light source for a signal light of an additional wavelength to be added to signal lights of wavelengths being used,
wherein the terminal apparatus transmits the signal light of the additional wavelength by gradually increasing a power of the signal light of the additional wavelength while receiving the monitoring and controlling signal sent from each trunk apparatus, and cancels an addition of the signal light of the additional wavelength by immediately decreasing a power of the signal light of the additional wavelength when the terminal apparatus receives the monitoring and controlling signal including an alarm sent from one of the trunk apparatuses, the alarm indicating that the addition of the signal light of the additional wavelength cannot be accepted by the one of the trunk apparatuses.